The present invention relates to the field of acoustic ink drop printers and, more particularly, to methods and apparatus for finely controlling the ink levels in the print heads of such printers.
Acoustic ink printing has been identified as a promising technology for manufacturing printers. The technology is still in its infancy but it may become an important alternative to ink jet printing because it avoids the nozzles and small ejection orifices that have caused many of the reliability and accuracy problems that are experienced with ink jet printers. The basic principles of this technology have been described in a series of U.S. Patent Nos., including: U.S. Pat. Nos. 4,308,547, "Liquid Drop Emitter", by Lovelady et al.; 4,751,530, "Acoustic Lens Array for Ink Printing", by Elrod et al.; 4,751,529, "Microlenses for Acoustic Printing", by Elrod el.; and 4,751,534, "Planarized Printheads for Acoustic Printing", by Elrod et al.
The print head in an acoustic ink printer comprises a pool of ink, a series of spatially aligned droplet ejectors and a mechanism for maintaining the surface of the ink at a desired level. When activated by an appropriate electrical signal, the droplet ejectors irradiate the surface of the ink with a beam of focused acoustic radiation thus forcing ink droplets to be ejected from the surface of the ink. The droplets are then captured on a nearby recording medium.
Experiments have shown that the position of the surface of the ink is critical to the success of the ink drop ejection process. The surface of the ink must remain within the effective depth of focus of the droplet ejectors. A great deal of effort has been devoted methods of controlling the surface of the ink. Elrod et al., in their U.S. patent application Ser. No. 07/287791 for "Acoustic Ink Printers Having Reduced Focusing Sensitivity" disclose a technique for suppressing the half wave resonances in the resonant acoustic cavities.
In another U.S. patent application filed on Dec. 19, 1986, entitled "Variable Spot Size Acoustic Printing", Elrod et. al suggest using a closed loop servo system for increasing and decreasing the level of the ink surface by utilizing an error signal which is produced by comparing the output voltages from the upper and lower halves of a split photodetector. The magnitude and sense of that error signal are then correlated with the free ink surface level via a laser beam reflected off the ink surface. While this is a workable solution to the problem, it is expensive to implement and the photodetector and laser beam must be kept in precise optical alignment.
Ink transport mechanisms have also been proposed in U.S. Pat. No. 4,801,953, "Perforated Ink Transports for Acoustic Ink Printing", by Quate; and U.S. Pat. No. 4,797,693, "Polychromatic Acoustic Ink Printing", by Quate. However, the free surface level control that is provided by these transport mechanisms is dependent upon the uniformity of the remote inking process and upon the dynamic uniformity of the ink transport process.
Finally, a perforated membrane has been devised which, in combination with a device for pressurizing the ink to an essentially constant bias pressure, maintains the surface of the ink more nearly within the effective depth of focus of the acoustic beams. The details of this membrane are revealed in U.S. patent application Ser. No. 07/358,752, "Perforated Membranes for Liquid Level Control In Acoustic Ink Printing", by Khuri-Yakub et al.
It can readily be seen that improvements in liquid level control would fill a long felt need in the field of acoustic ink drop printing.